The present invention relates generally to polyolefin production and, more specifically, to techniques that improve the catalyst activation in polyolefin production processes.
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
Polyolefins may be produced from various monomers, such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks. If one monomer is used for polymerization, the polymer is referred to as a homopolymer, while incorporation of different monomers creates a copolymer or terpolymer, and so on. Monomers may be added to a polymerization reactor, such as a liquid-phase reactor or a gas-phase reactor, where they are converted to polymers. In the liquid-phase reactor, an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and/or n-hexane, may be utilized as a diluent to carry the contents of the reactor. A catalyst may also be added to the reactor to facilitate the polymerization reaction. An example of such a catalyst is a chromium oxide containing hexavalent chromium on a silica support. Unlike the monomers, catalysts are generally not consumed in the polymerization reaction.
As polymer chains develop during polymerization, solid particles known as “fluff” or “flake” are produced. The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), copolymer content, comonomer content, modulus, and crystallinity. Different fluff properties may be desirable depending on the application to which the polyolefin fluff or subsequently pelletized fluff is to be applied. The reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, and so forth, may be affect the fluff properties.
In addition, the catalyst activity may affect the fluff properties. Catalyst activity may also affect the fluff production rate of the polymerization reactor. Catalyst activity may be defined as the mass of polymer produced per the mass of catalyst utilized. To increase the activity of the catalyst, raw catalyst may be activated or converted (e.g., changed oxidation state) in a catalyst preparation process (e.g., in an activator vessel with an external furnace) prior to its introduction into the polymerization reactor. In the case of a chromium (Cr) oxide catalyst, a catalyst activator upstream of the polymerization reactor may convert Cr3+ to Cr6+, for example, to improve the quality of the catalyst and to increase the activity of the catalyst.
In an industry where billions of pounds of polyolefin product are produced per year, small incremental improvements, for example, in catalyst activity, monomer yield, energy efficiency, diluent recovery, and so forth, can generate significant cost savings in the manufacture of polyolefins. For example, catalyst research has produced commercial catalysts with activity values that are orders of magnitudes higher than those of two to three decades ago, resulting in a striking reduction in the amount of catalyst utilized per pound of polymer produced, and also reducing the amount of downstream processing (and equipment) used to deactivate and/or remove residual catalyst in the polymer product. Further advances in the processing and activation of the catalyst may result in increased polymerization rates, increased polyolefin production rate, and improved polyolefin fluff properties.